1. Field of the Invention
This invention relates to a cylinder block, especially of an internal combustion engine, having at least one cylinder bore, and particularly to such a cylinder block having at least one cylinder bore formed by a cylindrical wall having a step-less plating coating, and a recessed cylindrical wall at an end, which cylinder block allows for preventing the plating coating from being detached at the edge caused by reciprocal movement of a piston. This invention also relates to a method for forming the above plating coating on the cylindrical wall.
2. Background of the Art
Heretofore, in order to lighten engines, cylinder blocks are made of an aluminum alloy in many cases, wherein the cylinder bore walls, i.e., piston-sliding surfaces, are plated with a metal such as nickel so as to improve durability of the walls.
FIG. 9 is a schematic vertical cross-sectional partial view showing a conventional plating apparatus into which a cylinder block is fitted during plating treatment. FIG. 10 is a schematic partial cross-sectional view showing a sealing-associated portion of the plating apparatus of FIG. 9, in which FIG. 10B is a schematic enlarged view of the circled portion of FIG. 10A. A high-speed plating apparatus 80 comprises an upper-surface supporting member 81 and a lower-surface supporting member 82 on which an upper surface 90a and a lower surface 90b of a cylinder block 90 are respectively placed, and a seal member 83 which is inserted using a shaft 83a into a cylinder bore 91 from the upper end. Upper and lower directions in the figures are lower and upper directions in the cylinder block in operation, since the cylinder block is set upside-down when fitted into the plating apparatus. The seal member 83 comprises an upper pressing member 83b, a lower pressing member 83c, and an O-ring 83d which is compressed between the upper pressing member 83b and the lower pressing member 83c. The upper pressing member 83b is connected to the shaft 83a. The upper-surface supporting member 81 is provided with a passage-forming cylindrical electrode 85 which is concentrically inserted into the interior of the cylinder bore 91 from the upper end of the cylinder bore 91 in such a way as to maintain a gap between the passage-forming cylindrical electrode 85 and the cylinder bore 91. Plating solution is introduced into the interior of the cylinder bore 91 through a plating solution passage 84 via a plating solution inlet 86, and while flowing along the inner wall of the cylinder bore 91, the plating solution is impressed with a voltage, thereby forming a plating coating 92 on the inner wall of the cylinder bore 91. The plating solution is then moved into a return passage 85a formed in the inside of the cylindrical electrode 85, and then returned to a treated-solution recovery passage (not shown). The flow of the plating solution is indicated with arrows in FIG. 9.
The cylinder bore 91 is constituted by a honing surface 91a, which is a surface subjected to honing treatment with a honing blade after plating, and a honing-release surface 91b, which is a recessed surface for releasing the honing blade, i.e., the diameter of the circumference of the honing-release surface 91b is greater than that of the honing surface 91a. Both the honing surface 91a and the honing-release surface 91b are formed when the cylinder block is cast. During operation of an engine, a piston (not shown) reciprocally slides upon the honing surface 91a in such a way that a piston skirt does not protrude from the lower end of the honing surface 91a even when the piston reaches the lower dead point. Accordingly, heretofore, only the honing surface 91a is plated by sealing the cylinder bore 91 at a lower position of the honing surface with a sealing member 83, as shown in FIGS. 10A and 10B.
The above embodiment is advantageous and economical in view of material costs incurred in metal plating, since only the area upon which the piston slides is plated.
However, despite the above advantage, the above embodiment has a drawback when the piston skirt descends slightly below the lower end of the honing surface. That is, the likelihood of detaching the lower edge of the plating coating 92 from the cylinder wall is significant, especially in a downsized engine in which the piston skirt 93a of the piston 93 descends beyond the lower end of the honing surface 91a when the piston 93 reaches the lower dead point, i.e., the height of the cylinder block is minimized, in order to realize a high-output downsized engine.